1. Field of the Invention
The present invention generally relates to the field of voice group services in mobile communication networks, and in particular to Voice Group Call Services (VGCS).
2. Discussion of Prior Art
Voice Group Call Service allows the establishment of a call between members of a group in numerous cells of the mobile telecommunication network simultaneously. These cells define the service area. All subscribers belonging to the group have the possibility to listen to the call in the service area. It is also possible to participate in the call individually. Service subscribers of the radio network can become group members on a network-wide basis to one or more groups pre-defined in the network by a corresponding group identification (group ID). The membership enables them to receive voice group calls associated with that group ID.
In addition, certain group members are entitled by their subscription to initiate Voice Group Calls (VGC). In addition to subscriber details in the Home Location Register (HLR), it is necessary for the mobile station (MS) to be aware of its group membership by storing details on the Subscriber Identity Module (SIM). This is required because it responds to notification messages which include only the group call relevant information, that is, no IMSI or TMSI details. Having become a group member, each service subscriber can set to active state or deactive state the group ID or any one out of his several group IDs on the SIM. In active state the subscriber can initiate VGCs to that group ID. When in deactive state the subscriber cannot make VGCs to the group and the mobile station ignores any notification for that group ID.
A group call area can be restricted to a single Mobile Switching Center (MSC) area or can exceed one MSC area (implementation option). A voice group call is initiated by a calling subscriber by a related man machine interface action for the service selection and the group ID dialed. The MSC in which the voice group call is initiated obtains by requesting a Group Call Register (GCR) the group call attributes. The GCR is functionality in the network containing the group call attributes. This GCR interrogation after call initiation also determines whether the MSC acts as group call Anchor- or as group call Relay-MSC. A group call Anchor-MSC is responsible for managing and maintaining a particular VGC. The group call Anchor-MSC is determined as the one controlling the cells of the group call area. For VGCSs where the group call area exceeds an MSC area, the group call Anchor-MSC is predefined in the network. A group call Relay-MSC controls cells of a group call area which are not under control of the group call Anchor-MSC for those voice group call services where the group call area exceeds one MSC area.
If the MSC is not the Anchor-MSC then the call will be “forwarded” from the Relay-MSC to the respective Anchor-MSC (information also delivered by GCR) and further “call-establishment” is done by the Anchor-MSC.
When a calling subscriber (talker) initiates a voice group call, one voice group call channel is established in each cell of the group call area and notifications for that call is sent in each of these cells. A voice group call channel is a downlink to be allocated in each cell of the group call area for a particular voice group call. All mobile stations being service subscribers for that Voice Group Call in one cell listens to the common downlink. All mobile stations of the listening service subscribers in one cell only listens to the same common downlink (voice group call channel). Of course it is possible to change the talker at any time, so that one of the listeners of the group becomes the talker and the previous talker becomes a listener.
The standards of Voice Group Call Service are specified in the technical specification ETSI TS 143 068, V5.2.0, December 2002: Digital Cellular telecommunication system (Phase 2+); Voice Group Call Service (VGCS); Stage 2; 3GPP TS 43.068 version 5.2.0 Rel. 5.
US patent publication 2003/0100326 A1 relates to a group location and route sharing system for communication units in a trunked communication system, wherein the communication units are engaged in a group voice call communication. Subscription and location data can be sent as a supplementary information to the group voice call communication by using an underlying packet transport layer.